This invention relates to the catalytic dehydrogenation of paraffinic and other hydrocarbons and most particularly to an endothermic catalytic dehydrogenation process involving on-stream dehydrogenation and off-stream catalyst regeneration and heating wherein the process performance is enhanced by a predehydrogenation reaction.
The catalytic dehydrogenation of paraffins is an endothermic, equilibrium-limited reaction. The extent of conversion is limited by thermodynamics with higher temperatures favoring higher conversions. In order for the dehydrogenation reaction to occur, heat must be supplied. In one type of prior art process, the catalyst is heated by contact with a heated gas, usually air. The hydrocarbon is then passed through the hot catalyst bed which supplies the heat for the endothermic reaction and lowers the catalyst temperature. At some point in time, the catalyst becomes too cool to sustain the reaction. The reactor is then taken off-stream and the catalyst is reheated by contact with the heated gas. The heated gas contains oxygen so that the heated gas also serves the additional purpose of regenerating the catalyst by the combustion of the carbonaceous deposits on the catalyst. This combustion also imparts further heat to the catalyst. After reheating and regenerating the catalyst and before putting the reactor back on-stream, the catalyst which has become oxidized must be reduced. This is done by passing a reducing gas such as hydrogen through the catalyst bed. This also supplies additional heat by the oxidation of the reducing gas. The reactor is then ready to be put back on-stream for the dehydrogenation reaction. Even further heat can be provided by injecting and combusting a fuel gas into the reheat air. Typically, the weight ratio of reheat air to hydrocarbon required to provide the process heat is between 4 and 8. In a typical cycle, the following are the percentages of heat inputs to the catalyst from the available sources:
The entire plant is composed of a multiplicity of reactors operating in a cyclic manner. Some of the reactors are operating with the hydrocarbon feed, some are operating with air in the reheat/regeneration mode and some are in the catalyst reduction mode.
The required air flow is set by both the desired heat requirements and by the necessity that excessive temperatures must be avoided during the reheat cycle to prevent catalyst deactivation. If the air flow were to decrease, then the injected fuel gas combustion would raise the temperature above that deactivation level. The air is provided to the process at a pressure of approximately 25 psig to overcome pressure drop through the bed which requires substantial compression equipment and power. Furthermore, the air that leaves the reactor during the reheat cycle still contains a substantial quantity of oxygen, approximately 10-15%, since high temperatures must be avoided. This represents a significant loss in process efficiency. The loss of energy up the stack is defined by the flow and temperature of the exhaust gas. The overall efficiency of the combustion process is defined by the total fuel input minus the stack losses divided by the total fuel heat input. The most efficient process is one that fully consumes all the oxygen available thus allowing for the firing of the maximum amount of fuel fired for a given flow of air and stack losses. If it is required to limit the temperature of the combustion gases as in this case where the temperature cannot exceed about 700xc2x0 C., then only a fraction of the oxygen can be utilized.
Plants are typically designed to be heat input limited. For a new plant, the cost of the air handling equipment represents a major capital cost. Once designed, a plant runs to the limit of the air handling equipment. Increasing capacity requires major capital expenditures for air handling equipment in addition to the other process equipment requirements. If additional heat could be provided for the reaction without requiring equivalent increases in air flow, the plant capacity and conversion level could be readily increased and the economics and performance of the process would see considerable benefits.
An object of the invention is to increase the performance of an endothermic catalytic dehydrogenation process without requiring additional air flow and compression. The invention involves the reaction of a preheated hydrocarbon stream in a catalytic prereactor with the partially dehydrogenated effluent from the prereactor then being reheated to the same preheat temperature prior to introduction into the main catalytic reactor. More specifically, one embodiment employs the effluent air from the main reactor being reheated and regenerated to supply heat for reheating the hydrocarbon effluent from the catalytic prereactor and also for regenerating the catalyst in the prereactor. In another embodiment, the heat for reheating the hydrocarbon effluent from the prereaction can be a separately fired heater.